1. Field of the Invention
The present invention relates to a foreign-object detection method for an injection molding machine, which method detects a foreign object which is caught between a movable mold and a stationary mold upon movement of a movable platen in a mold closing direction.
2. Description of the Related Art
Conventionally, in the field of an injection molding machine designed such that reciprocating motion produced by a drive unit composed of a servomotor and a ball-screw mechanism is transmitted to a movable platen via a toggle link mechanism, there has been known a foreign-object detection method for detecting a foreign object (e.g., a molded product) which is caught between a movable mold and a stationary mold upon movement of the movable platen in a mold closing direction (see, for example, Japanese Patent Application Laid-Open (kokai) No. 5(1993)-30609).
The patent publication discloses the following detection method. When a movable mold has arrived at a mold protection start position during mold closing operation, an NC controller outputs a torque limit command to thereby move the movable mold at low torque. When a difference (deviation) between a movement command signal and a feedback signal representing a position detected by a position detector exceeds a predetermined value (threshold value), a foreign object is judged to have been caught between the movable mold and the stationary mold.
In such a conventional detection method, detection is performed through comparison between a deviation and a threshold value, and therefore, setting of the threshold value is extremely important. That is, the deviation varies within a certain range even when the mold closing operation is performed properly. Therefore, when the threshold value is set to an excessively small value relative to the variation range, erroneous detection may occur, and when the threshold value is set to an excessively large value relative to the variation range, a foreign object cannot be detected properly and reliably, and in particular, a small foreign object cannot be detected at all. Therefore, in actuality, the threshold value is preset to a standard value, and an operator finely adjusts the standard value empirically and intuitionally (i.e., on the basis of personal know how).
However, such a conventional detection method requires fine adjustment (re-setting) of the threshold value whenever the mold is exchanged or whenever molding conditions are changed. Labor and time needed for such setting cannot be ignored, and reliable setting is impossible, because of variation in set values among operators.
An object of the present invention is to provide a foreign-object detection method for an injection molding machine, which method enables quick and easy setting of a threshold value used for detection of a foreign object and which enables reliable setting of a proper threshold value.
Another object of the present invention is to provide a foreign-object detection method for an injection molding machine, which method enables automatic setting of a threshold value to thereby enable precise setting without bothering an operator.
Still another object of the present invention is to provide a foreign-object detection method for an injection molding machine, which method increases a deviation generated upon catching of a foreign object to thereby enhance the foreign-object detection performance.
To achieve the above objects, the present invention provides a foreign-object detection method for an injection molding machine, comprising the steps of performing a trial mold closing operation; periodically detecting a physical value related to mold closing operation in a monitor region during the trial mold closing operation; obtaining a maximum deviation between the detected physical value and a preset theoretical value; adding the maximum deviation to a preset reference value to thereby determine a threshold value to be used for foreign object detection; periodically detecting the physical value related to mold closing operation in a monitor region during an actual mold closing operation; calculating a deviation between the detected physical value and the determined theoretical value; and judging that a foreign object has been caught when the deviation is in excess of the threshold value.